Pipes used as, for example, fuel lines in the motor vehicle sector usually have high stiffness. These lines must be thermoformed, since it is not possible to install them in an automobile in straight form. In this reshaping process, the previously straight pipe is permanently shaped so that it retains bends and curves at particular desired locations.
Thermoforming using polyethylene glycol is currently a very widespread method for the thermoforming of pipes. In this process, the pipe is generally laid into a template and, together with the template, is immersed into a bath of polyethylene glycol. This bath is heated to a temperature appropriate for the material of the pipe, so that the pipe is heated from outside and becomes soft. This heating reduces the internal resistance of the pipe to bending, i.e. the stresses introduced by bending are relaxed. After the actual thermoforming, the pipe, together with the template, is removed from the bath and cooled back to room temperature in a cooling bath, so that the intended shape of the pipe becomes fixed. A disadvantage of thermoforming with polyethylene glycol is the contamination of waste water caused by the process. Against a background of stricter legal requirements, this is becoming increasingly problematic, with the result that the process is made unattractive on cost grounds.
Another thermoforming process uses hot air, in which the pipe, together with the template, is held in a hot air oven. Because of the relatively poor transfer of heat between hot air and pipe, the cycle times are longer than for thermoforming using polyethylene glycol.
For thermoforming with superheated steam, the pipe is likewise placed into a template. Superheated steam is then conducted through the interior of the pipe, so that the pipe is heated from inside. After the resistance of the pipe to bending has been reduced, the pipe is cooled once again and the shape is thus fixed in the pipe. Thermoforming using superheated steam is, however, costly and time-consuming.
Another route is taken by contact heating. For this, heated metal templates are used to heat the pipe in the regions to be thermoformed. Here, too, long thermoforming times mean that the process is not sufficiently cost-effective
A common feature in all of these processes is that the heating of the pipe must largely occur via the conduction of heat, and this is time-consuming because of the poor thermal conductivity of plastics. In order to reduce thermoforming times, attempts to increase the temperatures of the heating media are often made. This, however, can often lead to overheating of the outer surface of the pipe, and even to burning, and this can have a negative effect on the properties of the pipe.
A further process which has been developed for thermoforming is the heating of the pipe using IR radiation. For this, the pipe is irradiated with light from the infrared spectrum. The radiation is absorbed to some extent by the pipe and converted into heat. However, a number of problems exist with this process. Firstly, it is difficult to irradiate the pipe uniformly around its circumference, for example because the pipes are placed in templates and thus are not freely accessible on all sides to the radiation. The heating of the pipe is therefore non-uniform, and this can have a negative effect on the properties of the pipe in service. A further great disadvantage of IR radiation is that the various molding compositions used for fuel lines absorb the IR radiation to different extents. For example, black pigmented pipe molding compositions have very good absorption, and the radiation is therefore absorbed almost immediately at the surface of the pipe. This, however, creates a particularly high risk of localized burning. Absorption of the radiation is generally significantly poorer in the case of unpigmented products, and this results in extended cycle times.